The present invention concerns still or motion-picture camera focussing systems of the type which emit radiation from the camera to the subject, and receive the reflected-back radiation. The focussing system's detector arrangement produces signals from which are ultimately derived focus-control data, used to drive an indicator which informs the user of the direction in which he should change the camera's present subject-distance setting, or else controlling the energization of an adjusting motor operative for automatically correcting the camera's state of focus.
With radiation-emissive focussing systems, it can easily happen that when the camera-to-subject distance is great, and/or the reflecting characteristics of the subject are low, the power of the reflected-back radiation incident upon the system's detector arrangement is too low to yield signals fit for processing and for the generation of focus-control data. Of course, this problem can be overcome by employing sufficiently high emitted-power levels, but this tends to increase the cost of the system and its energy consumption. The lower limit of the emitted-power level which can be employed is typically dictated by the noise level associated with the particular transducer design used for the detector arrangement and also by the noise present in the circuitry which processes the detector's output signals.